Server :: Ssh And Miscl - No Longer Keyless Connections And Use Tab

I have a server running on Ubuntu and I am having 2 problems: SSH-KEYGEN related-> I have several computers and I ssh into each, so I initially set up keyless connection via the following commands (more or less)

ssh-keygen -t rsa scp id_rsa.pub user@host:
#remote computer
touch .ssh/authorized_keys
chmod 600 .ssh/authorized_keys
cat id_rsa.pub >>.ssh/authorized_keys

everything works fine at first when, for example, connecting computer A to computer B. And it still works when connecting computer B to computer A. But when I introduce computer C, all of a sudden computer B gets fussy and no longer does keyless connections. Or if I have multiple users on computer A, lets call them A1-A9, then only A1 accepts keyless connections. My question is this: what is the rational between allowing keyless connection for some, but not all computers and/or users?

2) Tab related: when I create additional users, I can no longer use <tab> to complete a file or command name. It's like when one is in sftp, and the tab button actually inserts a full tab. Why does this occur